Process for producing metal fluoride single crystal

Abstract

A metal fluoride single crystal pulling apparatus that upward pulling initiation through termination, in the state of shallow melt capable of highly effective inhibition of scatterer formation, can perform stable growth of single crystal and can suppress any mixing of air bubbles and occurrence of crystal break during crystal growth, etc; and a process for producing a metal fluoride single crystal therewith. As a crucible for accommodating a melt of raw metal fluoride, use is made of a double structured crucible composed of an outer crucible and an inner crucible. In the upward pulling of single crystal, the accommodation depth of inner crucible relative to the outer crucible is increased in accordance with any decrease of melt accommodated in the inner crucible according to the growth of single crystal, so that the melt accommodated in the outer crucible is fed into the inner crucible to thereby maintain the amount of melt accommodated in the inner crucible within a given range. Further, at the position of an opening that upward opens any gap space between an external surface of outer crucible and an internal surface of inner crucible, or an interstice, situated below the opening, between a side wall internal surface of outer crucible and a side wall external surface of inner crucible, there is disposed a cutoff member that cuts off at least part of the position.

1. A process for producing a metal fluoride single crystal using an apparatus for pulling a metal fluoride single crystal,
 wherein, in the apparatus, a double structured crucible constituted by an outer crucible and an inner crucible accommodated in the outer crucible is provided in a chamber forming a single crystal growth furnace, 
 hollow portions of both the outer crucible and the inner crucible in the double structured crucible partially communicate with each other, 
 the double structured crucible can continuously change an accommodating depth of the inner crucible with respect to the outer crucible, 
 a single crystal pulling bar having a tip to which a seed crystal is attached so as to be used and suspended to be vertically movable is provided just above the hollow portion of the inner crucible in the chamber, and 
 a circular opening portion for upward opening a gap space formed by an internal surface of the outer crucible and an external surface of the inner crucible is provided between an internal surface of a sidewall of the outer crucible and an external surface of a sidewall of the inner crucible; 
 the process comprising: 
 accommodating a raw metal fluoride melting solution in each of the hollow portions of the outer crucible and the inner crucible, and then, once reducing the accommodating depth of the inner crucible with respect to the outer crucible to cause the melting solution accommodated in the inner crucible to flow toward the outer crucible side; 
 thereafter increasing the accommodating depth of the inner crucible with respect to the outer crucible again to feed, into the inner crucible, the melting solution in the outer crucible; and 
 carrying out the operation at least once and subsequently starting an operation for pulling a metal fluoride single crystal. 
 
     
     
       2. The process for producing a metal fluoride single crystal according to  claim 1 , further comprising:
 moving the single crystal pulling bar downward until the seed crystal attached to the tip of the single crystal pulling bar comes in contact with a surface of the melting solution accommodated in the inner crucible and then pulling the single crystal pulling bar gradually to grow a metal fluoride single crystal; 
 increasing the accommodating depth of the inner crucible with respect to the outer crucible in the growth of the metal fluoride single crystal, according to a decrease of the melting solution accommodated in the inner crucible with the growth of the metal fluoride single crystal; and 
 supplementing the melting solution accommodated in the outer crucible into the inner crucible in such a manner that an amount of the melting solution in the inner crucible is maintained within a certain range. 
 
     
     
       3. The process for producing a metal fluoride single crystal according to  claim 2 , wherein the accommodating depth of the inner crucible with respect to the outer crucible is increased in such a manner that the amount of the melting solution in the inner crucible has a depth of 3 cm or more and is maintained within a range of at least 0.65 time as great as a diameter of a straight barrel portion of the single crystal during the pulling of the metal fluoride single crystal. 
     
     
       4. The process for producing a metal fluoride single crystal according to  claim 1 , wherein a floating solid impurity mixed in the melting solution accommodated in the inner crucible is discharged to the outer crucible side by the operation, and the solid impurity floats over a surface of the melting solution accommodated in the outer crucible. 
     
     
       5. The process for pulling a metal fluoride single crystal according to  claim 1 , wherein the inner crucible is provided with a communicating hole for causing hollow portions of both the outer crucible and the inner crucible to communicate with each other, wherein the communicating hole is formed on a lowermost end of the sidewall or a bottom wall when the bottom wall is horizontal, or below a position in which an inside diameter of the bottom wall is equal to or smaller than ¼ of an inside diameter of the inner crucible when the bottom wall takes a downward convex shape. 
     
     
       6. The process for pulling a metal fluoride single crystal according to  claim 1 , wherein a shielding member for shielding at least a part of a portion between the internal surface of the sidewall of the outer crucible and the external surface of the sidewall of the inner crucible is provided in the opening portion or a position between the internal surface of the sidewall of the outer crucible and the external surface of the sidewall of the inner crucible below the opening portion.